Radionuclide ventriculography can accurately track changes in left ventricular (LV) volumes at rest and during vigorous aerobic exercise (EX). Although age-associated changes in LV volumes and ejection fraction (EF) have been shown during EX in prior studies, the role of age changes in cardiovascular fitness on these LV parameters during EX has not been assessed. To determine the effect of age on EX LV volumes, independent of physical fitness level, we obtained gated cardiac blood pool scans during graded maximal upright cycle EX in 95 healthy men ages 23-86 yr. At maximal effort in the overall group, LV end-diastolic volume index (EDVI) and end-systolic volume index (ESVI) increased with age (r = 0.37, p<.01 and r = 0.46, p<.001 re-spectively) while LVEF declined with age, r = - 0.46, p<.001 and stroke volume index (SVI) was unrelated to age. In a subset of 49 men of homogeneous fitness (maximal workload 125-150 watts), EDVI and ESVI increased and LVEF decreased with age as in the overall sample, but SVI increased with age (r = 0.39, p<.01) due to an accent- uated increase in EDVI (r = 0.51, p<.001). Age differences in the cardiac response to beta-adrenergic stimulation may underlie aging changes in cardiovascular EX hemodynamics. One way to test this hypothesis is to administer a beta-adrenergic blocking drug to normal individuals across a broad age range and compare the EX re-sponse to that in the unblocked condition The response to strenuous aerobic EX is mediated in large part by beta-adrenergic activation, the efficiency of which de-clines with advancing age. To ascertain whether acute beta-blockade would attenu-ate these normative age changes in cardiovascular EX performance, we performed maximal cycle EX in a group of BLSA men after intravenous propranolol. In 25 men ages 28-72 yr given intravenous propranolol, EDVI at peak workload declined with age (r= -0.20) causing an age-associated decline in SVI (r= 0.48, p<0.05) not pre-sent in the unblocked men. The decline in heart with age in propranolol-treated men was blunted (0.46 beats/min/yr) compared to controls (1.09 beats/min/yr). Max-imal LVEF declined with age similarly in the presence (r= -0.50, p<.01) and the absence (r= -0.45, p<.0001) of beta-adrenergic blockade. The primary reason for the slope shifts in the age regressions in the presence of propranolol was a large increase in EDVI and SVI and a large decrease in HR in younger men. Thus, while the age-associated decline in maximal LVEF is unaffected, acute beta-blockade reverses the age-associated ventricular dilatation during upright cycle and blunts the de-cline in maximal HR. The net effect is an accentuated decline of maximal CI with age.